Diamagnetic response of cylindrical normal-metal–superconductor proximity structures with low concentration of scattering centers

We have investigated the diamagnetic response of composite NS proximity wires, consisting of a clean silver or copper coating, in good electrical contact to a superconducting niobium or tantalum core. The samples show strong induced diamagnetism in the normal layer, resulting in a nearly complete Meissner screening at low temperatures. The temperature dependence of the linear diamagnetic susceptibility data is successfully described by the quasiclassical Eilenberger theory including elastic scattering characterized by a mean-free path l. Using the mean-free path as the only fit parameter we found values of l in the range 0.1–1 of the normal metal-layer thickness $d_N$, which are in rough agreement with the ones obtained from residual resistivity measurements. The fits are satisfactory over the whole temperature range between 5 mK and 7 K for values of $d_N$ varying between 1.6 and 30 μm. Although a finite mean-free path is necessary to correctly describe the temperature dependence of the linear-response diamagnetic susceptibility, the measured breakdown fields in the nonlinear regime follow the temperature and thickness dependence given by the clean limit theory. However, there is a discrepancy in the absolute values. We argue that in order to reach quantitative agreement one needs to take into account the mean-free path from the fits of the linear response.